Many vehicles are used over a wide range of vehicle speeds, including both forward and reverse movement. Some types of engines, however, are capable of operating efficiently only within a narrow range of speeds. Consequently, transmissions capable of efficiently transmitting power at a variety of speed ratios are frequently employed. When the vehicle is at low speed, the transmission is usually operated at a high speed ratio such that it multiplies the engine torque for improved acceleration. At high vehicle speed, operating the transmission at a low speed ratio permits an engine speed associated with quiet, fuel efficient cruising.
Some transmissions, called discrete ratio transmissions, are configured to establish a finite number of speed ratios between an input shaft and an output shaft. When the currently selected ratio is no longer appropriate, a discrete ratio transmission must shift to a different one of the available speed ratios. Other transmissions, called continuously variable transmissions (CVTs), are capable of establishing any speed ratio between lower and upper limits. CVTs are capable of making frequent fine speed ratio adjustments which are not perceivable by vehicle occupants.
Many transmissions use hydraulically actuated friction clutches to establish various power flow paths. Hydraulic actuation is suited for clutches that selectively couple rotating elements to one another because pressurized hydraulic fluid can be routed from a stationary housing to rotating components between seals. Therefore, the hydraulic actuator can rotate with one of the rotating elements. When there are multiple hydraulically actuated clutches, the clutches often share an engine drive pump and share many of the valve body components used to regulate the pressure.
Hybrid vehicle transmissions improve fuel economy by providing energy storage. In a hybrid electric vehicle, for example, energy may be stored in a battery. The battery may be charged by operating the engine to produce more power than instantaneously required for propulsion. Additionally, energy that would otherwise be dissipated during braking can be captured and stored in the battery. The stored energy may be used later, allowing the engine to produce less power than instantaneously required for propulsion and thereby consuming less fuel.